1. Field of the Invention
The present invention relates to a method of manufacturing a titanium alloy with high strength and high formability, and more particularly a titanium alloy provided with ultrafine grains with high strength/high formability through rolling that gradually changes a forming temperature in accordance with the strain under relatively low strain (strain of 2.5 or less) without using severe plastic deformation known in the art, and a method of manufacturing the titanium alloy.
In detail, the present invention relates to a titanium alloy having ultrafine grains with high strength/high formability by making the final temperature, at which forming is finished, a plastic instability temperature or less of an initial lamellar structure (martensite) while gradually decreasing a forming temperature into an optimized condition, by using a principle that formability increases with an increase in a fine spheroidized structure during deformation, after starting deformation at the beta transformation temperature or less and the plastic instability temperature or more of an initial lamellar structure (martensite), without simply decreasing the forming temperature during forming, and a method of manufacturing the titanium alloy.
2. Description of Related Art
A titanium alloy is typical lightweight metal and has high specific strength and excellent corrosion resistance, so that it can be used for various fields, such as a material for the aerospace industry, a material for the chemical industry, a material for bio-implant, and a material of sports products. Since the titanium alloy has a superplastic property, it is possible to reduce the weight of a product and the machining cost by performing superplastic forming. Therefore, it is possible to create a large added value by applying the titanium alloy to industries.
It has been known that superplastic forming of the titanium alloy is possible only by processing the titanium alloy generally at a high processing temperature of 850° C. or more with strain rate of 0.001/sec or less. However, since the superplastic property is largely influenced by a fine structure, superplastic forming can be performed on a titanium alloy composed of fine grains at a lower processing temperature or higher strain rate than the related art.
Accordingly, with the development of nanotechnology, a research about a method of manufacturing a titanium alloy with fine grains has been actively conducted.
Meanwhile, there are methods, such as powder metallurgy, mechanical alloying, rapid solidifying, recrystallizing, forging, rolling, and drawing, as the methods of manufacturing a material with fine grains.
However, it is difficult to manufacture a material having a sufficient size, using the methods, and a large amount of pores may be formed inside. Further, as the size is limited or the strain increases, in of recrystallized grains, the cross-sectional area decreases, so that it is difficult to give a large amount of deformation and there is a specific limit in making the grains fine. Therefore, it is difficult to practically apply making grains fine with the method.
Recently, severe plastic deformation that makes grains fine without generating pores inside by performing severe plastic deformation without specific heat treatment has been proposed. There are HPT (high pressure torsion) and ECAP (equal channel angular pressing), as the severe plastic deformation.
The HPT machining is a method of generating shear deformation under a high pressure and has a problem in that machining can be performed with a high speed at a room temperature, while the size of the material is limited and the thickness and the fine structure of the material are not uniform.
The ECAP machining is a method of generating shear deformation of a material by putting in the material to an L-shaped channel and is economical because it is possible to perform forming with existing equipment and increase the scale. Further, since the cross-sectional area of the material does not decrease even if the amount of machining increases, it is possible to provide the material with a large amount of deformation.
However, the sizes of specimens produced by the current severe plastic deformation are so small that it is very restrictive to industrially produce and use a titanium alloy with ultrafine grains.
Further, since the severe plastic deformation of the related art requires large strain (4 to 8), there is a problem in that it is difficult to make grains ultrafine by using rolling or extruding equipment of common coMPanies.